In the manufacture of glass products, particularly glass fiber products, glass is continuously melted in a large furnace capable of raising the temperature of the glass to temperatures on the order of 2600.degree. F. (1426.degree. C.). Glass batch material is usually supplied either to one end of the furnace, or to the top surface of the molten glass, in the case of an electric furnace. Heat is supplied to the molten glass and to the unmelted batch material by burners positioned above the batch material in a fossil fueled furnace, and is supplied by electrodes positioned within the molten glass in an electric furnace.
Subsequent to melting, the molten glass slowly travels through the furnace and passes through the furnace exit or throat to the forehearth. The movement of the glass is usually effected by convection currents, bubblers and glass pull effects. In order to provide glass of uniform consistency, the glass is made to travel slowly through the melter and through the forehearth. The forehearth supplies the molten glass to apparatus suitable for forming the glass products, such as bushings for producing continuous glass fibers. The apparatus for producing the glass products is generally sensitive to temperature changes in the molten glass supplied thereto. Therefore, it is extremely important that the temperature of the molten glass supplied to such apparatus not change over time.
There are several factors which tend to cause disruptions in the uniformity of the glass temperature in the forehearth. Changes in the pull rate, the batch distribution process, or in the batch composition itself, can cause non-uniformities. Changes in the bubbler, or in the level of the molten glass within the furnace, can also cause problems with uniformity of temperature. Also, heat losses from the molten glass to the atmosphere and to the walls of the furnace can vary, thereby producing non-uniformities in the temperature of the glass by the time it reaches the forehearth.
One aspect of the furnace controls of many present furnaces is that there is a considerable time lag between the time changes are made in the amount of heat Q provided to the furnace and the time at which such changes can be perceived at the furnace exit or throat. Almost all glass furnace control systems rely on temperature measurements at various positions within the furnace to control the amount of heat provided to the furnace. Temperature measurements are usually taken within the combustion chamber of the furnace, at the exit or throat of the furnace, and at several other locations within the molten glass.
Current methods of controlling the amount of heat Q provided to the furnace in response to temperature measuremens are not entirely satisfactory. Temperature measurements in some locations are more sensitive to changes in the molten glass temperature, and are better predictors of the final temperature of the molten glass when it reaches the furnace exit or throat. A controller which controls a furnace just based on temperature measurements fails to recognize the effects of different thermal charges, or entropy E, being produced in the melting process.